Mixed greens Kangaroos have survived through millennia of dramatic environmental change in Australia partly by having a very flexible diet, a new analysis of fossilised marsupial teeth suggests.

The research, published today in the journal PLOS ONE, reveals that during the increasingly arid Pliocene epoch, an ancestor of modern kangaroos lived, just as its modern cousins, on a mixed diet of leaves and grass.

"It was ecologically flexible enough to deal with increasing arid conditions since the Pliocene," says vertebrate palaeo-ecologist Dr Julien Louys of the University of Queensland.

"It could quite happily subsist on just eating leaves, it could quite happily subsist on just eating grass."

Fossil evidence suggests that kangaroos have been around in Australia for around 25 million years.

Back then the continent was covered in rainforest which kangaroos had to survive predominantly on browsing tree leaves.

During the Pliocene however, 2.5 to 5 million years ago, the rainforests retreated towards the coast and north, and there was an increase in open sclerophyll woodlands and grasslands.

Kangaroos evolved into a number of groups including the ancestors of modern grazing kangaroos, such as Macropus, which are adapted to more open spaces.

Ancient roo diet

Louys says understanding the Pliocene environment in greater detail is important in understanding what different groups of kangaroos ate, and why some species went extinct, while others thrived.

He and colleagues investigated this question by analysing isotopes of oxygen and carbon in fossilised marsupial teeth from the Chinchilla Sand deposits of southeast Queensland.

The samples studied included teeth from three kangaroo species, including Macropus (a genus that has survived through to today) and two extict animals Protemnodon and Troposodon.

The ratio of carbon-12 and carbon-13 istopes in the teeth shed light on the relative amount of trees and shrubs, and grasses. The ratio of oxygen-16 and oxygen-18 gave an indication of the amount of water around.

The researchers then compared their results with isotope analyses of teeth from modern kangaroos from known regions of Australia, to work out what the Pliocene animals were eating and drinking.

Louys says the carbon isotopes suggested that the animals lived in a mosaic of grassland and forest.

While Troposodon ate only leaves, Macropus and Protemnodon ate a mixture of leaves and grass.

Louys suggests this means that a mixed diet was an important part of Marcropus' survival through various environmental changes to the present.

"The fact that they were around in the Pliocene and they are around today means they've been able to deal with everything that nature's been able to throw at them," he says.

One surprise from the analysis of oxygen isotopes is that the Pliocene climate of southeast Queensland was not as dry as previously thought.

"It appears the animals were living in something more tropical or more temperate than Chinchilla is today," says Louys.

He says other evidence suggests it was probably a more tropical environment, since dense rainforest persisted in the general area to the mid-Pleistocene.